Fluororesins are excellent in chemical stability and thermal stability, and have self-lubricating properties which are responsible for advantageous characteristics such as low friction coefficients. Despite their excellent properties, the fluororesins are used in limited applications because of their much higher prices than those of common resins.
Additionally, their chemical stability makes it difficult to produce a laminate of a fluororesin sheet or film bonded to another material with an adhesive so as to reduce the amount of fluororesin to be used. Therefore, the use of fluororesins has not been spread to, for example, closures (or plugs, or caps) for drug containers to which their chemical stability is advantageous.
In this context, Patent Literature 1 discloses a technique to improve the hydrophilicity of the surface of a fluororesin molding. Specifically, the surface of a fluororesin molding is roughened by plasma irradiation, and fluorine atoms on the surface of the molding are replaced by implanted plasma ions in order that the fluororesin can be bonded to another material with an adhesive.
However, such a method as disclosed in Patent Literature 1 which hydrophilizes the surface by replacing inactive atoms on the surface with other atoms by plasma treatment is known to have a problem with durability of the surface modification effect (hydrophilicity) which is caused by factors such as hiding of functional groups on the treated surface (modified surface) inside the resin due to thermal fluctuations of the polymer, and dissociation of low-molecular weight hydrophilic moieties formed on the surface.
As the surface modification effect decreases over time, the usable lifetime (shelf life) of fluororesin-based products which are surface-modified by the method of Patent Literature 1 is limited to a short period. If, because of poor demand forecast, the production in a subsequent process in which a fluororesin-based product prepared in the previous step is used as a material turns out lower than the forecast, a stock of fluororesin-based products may then become faulty products (defective products).